This invention relates to a technology for preventing surface luster degradation that sometimes occurs during production of hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet using a continuous hot-dip plating line.
JPA. 10-226865, JPA. 10-306357, U.S. Pat. No. 6,235,410 (U.S. Pat. No. 6,235,410 B1) and U.S. Pat. No. 6,379,820 (U.S. Pat. No. 6,379,820 B1) owned by the applicant of this application teach that a plated steel sheet having corrosion resistance and surface appearance sufficient to meet the requirements of an industrial product can be obtained manufacturing a hot dip Zn-base plated steel sheet using a hot-dip Znxe2x80x94Alxe2x80x94Mg-system bath composed of Al: 4.0-10%, Mg: 1.0-4.0%, Ti: 0.002-0.1%, B: 0.001-0.045% and the balance of Zn and unavoidable impurities, and imparting to the plating layer a metallic structure including [primary crystal Al phase] and [Zn single phase] in a matrix of [Al/Zn/Zn2Mg ternary eutectic crystal structure]. The production conditions for obtaining this metallic structure are set out in the published specifications.
In the course of working with such a relatively high Al- and Mg-content hot dip Z-base plated steel sheet in ensuing production processes, the inventors discovered that, depending on the production conditions, the surface luster of the plating layer deteriorated within two to three days. This surface luster degradation of the plating layer discolors the aesthetically pleasing plated surface just after plating to a somewhat blackish interference color with passage of time (as quickly as 2-3 days but sometimes taking 4-7 days). The degree of outermost surface layer discoloration (a kind of blackening) can be quantified by measuring the surface brightness. For example, a plating surface that exhibited a brightness (L value) of around L=82 just after manufacture degrades to around L=72 in 7 days. Although this decline in L value does not degrade the corrosion resistance property of the product and does not impair the physical and chemical qualities of the plated steel sheet, it is undesirable from the point of surface appearance. The fact that the surface luster degradation is not uniform over the plating surface, but takes the form of splotchy discoloration, particularly detracts from the aesthetic appeal.
This degradation of surface luster can be considered to be peculiar to plated steel sheets having relatively high Al and Mg content like the hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheets mentioned above. The degree of oxidation of Mg concentrated in the outermost surface layer of the plating and the oxidized state of the surface layer Al are most likely complexly involved as causes of the degradation. No reports have been published to date regarding the mechanism of the surface luster degradation occurring in such a hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet or regarding a method for inhibiting the degradation.
An object of the present invention is therefore to provide means for inhibiting the degradation of surface luster that appears as a problem peculiar to hot-dip Znxe2x80x94Alxe2x80x94Mg plated steel sheet.
The inventors carried out extensive experimentation and research directed to overcoming the foregoing problem. As a result, we determined that the degradation of the surface luster of hot-dip plated steel sheet is caused by the reaction behavior between the plating layer surface and water during water cooling in the cooling step following plating, that one way to prevent the surface luster degradation is to xe2x80x9ccontrol the contact temperature between the plating layer and the water streamxe2x80x9d in the water cooling step after plating layer solidification, that, by extension, it is effective to suitably control the xe2x80x9cstrip temperature during contact with the water stream,xe2x80x9d and that it is also effective to stabilize the oxidation state of plating surface layer Al and Mg by incorporating a small amount of a suitable xe2x80x9creadily oxidizing elementxe2x80x9d in the plating bath. By xe2x80x9cstrip temperature during contact with the water streamxe2x80x9d is meant the strip temperature at the time cooling is conducted while forming a water film on the plating layer surface in the cooling step after completion of plating layer solidification. Specifically, it is the plating layer temperature when a water stream is passed onto the completely solidified plating layer and the plating layer is being cooled with a water film formed on the surface of the plating layer.
In other words, according to the inventors"" findings, when steel strip is continuously immersed in and withdrawn from a hot-dip Znxe2x80x94Alxe2x80x94Mg-system bath of the foregoing description and the steel strip is thereafter continuously passed through a water quenching zone that effects cooling while bringing the completely solidified plating layer surface into contact with a water stream (while providing onto the plating layer surface an amount of water enabling momentary formation of a water film on the plating layer surface), the aforesaid degradation of plating layer surface luster can be inhibited by controlling the strip temperature on the water quenching zone entry side to lower than 105xc2x0 C.
The strip temperature on the water quenching zone entry side, while depending on the equipment conditions, depends strongly on the sheet thickness. When the strip thickness is great, controlling the strip temperature on the water quenching zone entry side to lower than 105xc2x0 C. may not be easy. It was found that in such a case, similar inhibition of surface luster degradation can be achieved without lowering the strip temperature on the water quenching zone entry side to lower than 105xc2x0 C. (i.e., with the temperature at 105xc2x0 C. or higher) by adding to the plating bath a small amount of an element that has a very powerful affinity for oxygen (that is a readily oxidizing element) and that has a stabilizing effect on Al oxides. Such elements include, for example, the rare earth elements, Y, Zr and Si. In actual practice, it suffices to add about 0.002-0.05 mass % of the readily oxidizing element.
Thus, the present invention provides a method of producing a hot-dip Zn plated steel sheet excellent in luster-retaining property comprising a step of continuously immersing and withdrawing steel strip into/from a hot-dip Zn base bath containing Al: 4.0-15 mass % and Mg: 1.0-4.0 mass %, thereby forming a plating layer thereon, and a step of thereafter continuously passing the steel strip through a water quenching zone that effects cooling while bringing the completely solidified plating layer surface into contact with a water stream, degradation of the plating layer luster being inhibited at this time by controlling strip temperature on the water quenching zone entry side to lower than 105xc2x0 C. The hot-dip Zn base bath is preferably composed of, in mass %, Al: 4.0-15%, Mg: 1.0-4.0%, Ti: 0.001-0.1%, B: 0.001-0.045% and the balance of Zn and unavoidable impurities.
The present invention further provides a method of producing a hot-dip Zn plated steel sheet excellent in luster-retaining property comprising a step of continuously immersing and withdrawing steel strip into/from a hot-dip Zn base bath containing Al: 4.0-15 mass % and Mg: 1.0-4.0 mass % and added with 0.002-0.05 mass % of at least one readily oxidizing element selected from among rare earth elements, Y, Zr and Si, thereby forming a plating layer thereon, and a step of thereafter continuously passing the steel strip through a water quenching zone that effects cooling while bringing the completely solidified plating layer surface into contact with a water stream, degradation of the plating layer surface luster being inhibited at this time by controlling strip temperature on the water quenching zone entry side to not lower than 105xc2x0 C. and not higher than 300xc2x0 C. The hot-dip Zn base bath is preferably composed of, in mass %, Al: 4.0-15%, Mg: 1.0-4.0%, Ti: 0.001-0.1%, B: 0.001-0.045%, at least one readily oxidizing element selected from among rare earth elements, Y, Zr and Si: 0.002-0.05%, and the balance of Zn and unavoidable impurities.
The present invention further provides, as a hot-dip Zn plated steel sheet obtained the foregoing methods, a hot-dip Zn plated steel sheet excellent in luster-retaining property obtained by hot-dip plating a steel strip using a hot-dip plating bath comprising, in mass %,
Al: 4.0-15% and
Mg: 1.0-4.0%,
optionally,
Ti: 0.001-0.1% and
B: 0.001-0.045%,
preferably,
at least one readily oxidizing element selected from among rare earth elements, Y, Zr and
Si: 0.002-0.05%, and
the balance of Zn and unavoidable impurities, thereby forming a plating layer thereon. The plating layer has a metallic structure including [primary crystal Al phase] in a matrix of [Al/Zn/Zn2Mg ternary eutectic crystal structure] or [primary crystal Al phase], [Zn phase] and/or [Zn2Mg phase] in a matrix of [Al/Zn/Zn2Mg ternary eutectic crystal structure].